Production of esters



Patented Oct. 8, 1946 PRODUCTION OF ESTERS John E. Mahan, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application June 24, 1944, Serial No. 542,011

6 Claims.

This invention relates to the production of esters of carboxylic acids by the novel interaction of olefinic hydrocarbons with nitriles (cyanides) in the presence of an inorganic acid. In one of its more specific aspects, this invention relates to the production of such esters by a single-stage operation involving the interaction of a mono-olefin with a saturated or unsaturated nitrile (alkyl or alkenyl cyanides) in the presence of an inorganic acid.

Numerous methods for the preparation of esters are known. According to one method, carboxylic acids are reacted with olefins in the presence of catalysts such as salts of mercury, copper and other heavy metals, relatively non-volatile inorganic acids such as phosphoric acid and sulfuric acid and their salts, and other similar materials. In recent years the use of olefins as a source of esters has attracted considerable industrial attention largely because of th increasing demand for esters for use as lacquer solvents, emulsifying and wetting agents, detergents, plasticizers and the like. Recommended olefins for the production of such esters have ranged from ethylene to octenes and higher homologs. The manufacture of gasoline by cracking processes has furnished a ready source of butenes and lower olefins at a relatively low cost. In addition, substantially mono-olefinic hydrocarbons of relatively high molecular weights are obtained from polymerization processes in which butenes, mixtures of butenes and propylenes, and similar materials are used as charge stocks.

The utility of the olefin-carboxylic acid reaction as a method for the production of esters has not been fully realized heretofore, possibly because of the lack of economical processes for the preparation of acids. Acids are readily obtained by the oxidation of alcohols, aldehydes and ketones in the presence of appropriate oxidizing agents. They may also be produced from alkyl chlorides, ethers and esters by suitabl reactions. The hy=- drolysis of nitriles has also been used as a source of acids. Catalytic oxidation of crude petroleum, petroleum distillates and residues, shale oils and tars, also yields acids as products. Olefins yield acids when treated with carbon monomde and steam or when oxidized in the presence of ozone. In most instances, however, all such general methods for the production of acids suffer from such limitations as low yields, severe reaction conditions, or relative non-availability of starting materials. Furthermore, the number of process steps involved in the preparation and isolation of certain acids imposes serious limitations from an economic viewpoint,

The value of nitriles as intermediates for the production of a wide variety of chemical products, such as acids, aldehydes, ketones, amines and the like, has long been recognized. However, it has not been known that nitriles and olefins undergo interaction in the presence of aqueous solutions of inorganic acids with the resultant formation of esters.

It is an object of the present invention to provide a process for the production of esters at low cost.

Another object of the present invention is to provide a process whereby esters of carboxylic acids may be produced by the interaction of nitriles with olefinic hydrocarbons.

A further object of the invention is the provision of a single-stage process for the production of esters by the interaction of nitriles of either saturated or unsaturated carboxylic acids with mono-olefins in the presence of a solution of sulfuric or other inorganic acid.

Further objects and advantages of the invention, some of which are referred to specifically hereinafter, will be obvious to those skilled in the art to which the invention pertains.

I have found that nitriles and olefins interact in the presence of an inorganic acid such as sulfuric acid under reaction conditions specified hereinafter. This reaction, which leads to the formation of esters, has not heretofore been described in the art. While the mechanism of the reaction is not clearly understood, it might appear that a step-wise reaction occurs which involves the hydrolysis of the nitrile to an acid in the first step and the subsequent addition of the acid to the double bond of the olefin in the second step. However, I have found that the substitution of the corresponding carboxylic acid, which is the intermediate in such reaction, for the particular nitrile does not result in the production of an ester under the same reaction conditions. The exact mechanism of the reaction is not obvious nor has it been determined as yet, but the ultimat result may possibly and probably does conform to the following equation:

i it R-CN X-o=o-Y 21110 RCOC-C-Y NH:

H n H H in which R is an alkyl or alkenyl radical and X and Y are hydrogen or the same or difierent alkyl radicals.

In accordance with a preferred embodiment of the process of the present invention, controlled proportions of the selected nitrile and olefin are brought into intimate contact with an aqueous solution of an inorganic acid such as sulfuric acid, phosphoric acid and the like at a suitable temperature. During the course of the reaction the mixture is vigorously agitated, after which it is passed to a quiescent zone where an aqeuous bottom layer separates and is discarded. The ester is recovered from the water-insoluble layer, preferably by washing said layer free of acid components with an aqueous sodium or potassium bicarbonate solution, drying with any suitable conventional drying agent, and subjecting the resulting dried material to distillation.

The reactants which are used in the process of the present invention comprise olefinic hydrocarbons and nitriles in the presence of a solution of an acid such as sulfuric acid. Low-molecularweight olefins such as ethylene, propylene, b-utylenes and amylenes, as well as branched monoolefins having higher molecular weights, such as those containing up to about 16 carbon atoms or more, may be used. These olefins may be derived from any convenient source, such as by thermal and catalytic cracking of petroleum gases and dis tillates, polymerization reactions and the like. Aliphatic as well as cycloaliphatic olefins may be reacted in accordance with the process of the invention. While relatively pure olefins are ordinarily preferred, it is often advantageous to carry out the reaction with mixtures of olefinic materials, with or without subsequent purification of the esters, depending upon the esters formed and the uses for which they are intended. The nitriles may be either saturated or unsaturated and may be prepared by any of the methods known to the art. Typical nitriles are propionitrile, butyronitrile, acrylonitrile, pentenenitriles and the like. The nitriles may contain halogen or other substituents, for example, 2 and 3-chloropropionitriles and 2, 3 and l-chlorobutyro-nitriles. When sulfuric acid is used in the present process, a solution containing about 65 per cent acid is preferred.

Reaction temperatures are ordinarily maintained at a level preselected to give an adequate reaction rate but, in general, relatively low temperatures, which favor ester formation, are used. They may vary from approximately to approximately 200 C. with temperatures between the range of approximately 40 to approximately tures in the range of approximately to approximately C. are most generally applicable. The temperature chosen will be dependent upon the olefin feed and is an important factor in the present process since it has been found that ester formation does not occur at any substantial rate above certain limits.

Reaction pressures are governed largely by the nature of the olefin used in the process and may extend from atmospheric pressure to about 300 pounds per square inch or higher. If normally gaseous olefins, such as ethylene or propylene, are used, an inert saturated hydrocarbon diluent, such as normal pentane, is preferably employed in admixture with the olefin in the ratio of about 15 per cent of olefin to 85 per cent diluent, and the pressure is maintained in the range of approximately 225 to approximately 300 pounds per square inch. In all cases, liquid-phase operation is preferred, and suflicient pressure is employed to maintain the reactants in a liquefied condition at the reaction temperature. Inert diluents are preferably used only in such cases where they are advantageous in holding the operating pressure within a convenient range.

In the practice of the process of the present invention, it is ordinarily preferred to have a. molecular excess of one of the organic reactants. Accordingly, the ratio of olefin to nitrile is usually kept at about 2 to 1. However, successful results have been obtained when the nitrile is used in excess.

The quantity of per cent sulfuric acid employed is adjusted to give a molecular excess of sulfuric acid (H2804) with respect to the nitrile. One of the functions of the acid, besides that of acting as catalyst, appears to be that of absorbing and reacting with any ammonia, if such is formed in the reaction. Other inorganic acids are used at suitable concentrations which can be readily determined by experiment and are likewise used in stoichiometrical excess of the nitrile. Water appears to be essential and is preferably added with the acid but not in such amounts as to dilute the acid so that the acid is no longer an effective catalyst in the reaction. Other acids which may be used as catalysts in the reaction are phosphoric acid, benzenesulfonic acid and the like. Preferably the acids used are such as do not form compounds with the olefin that are more stable than any of those possibly formed with sulfuric acid or that react with the olefin in other manners. For this reason, hydrochloric acid, nitric acid, and certain other acids are usually not desirable.

The time of contact between catalyst and reactants is subject to rather wide variation and is dependent on the nature of the olefin and nitrile as well as on the other reaction conditions. In some instances, maxmum yields are realized with a reaction time of a few hours, while in other instances, involving more refractory intermediates, several days may be required.

The process of the present invention is illustrated in the following specific examples. However, since numerous other process modifications and variations will be obvious to those skilled in the art in the light of the foregoing disclosure, no undue limitations are thereby intended.

Example I The preparation of an alkyl propicnate was effected from propionitrile (ethyl cyanide) and an olefinic polymer consisting essentially of dodecenes (0121124) in the presence of sulfuric acid. A mixture of 772 g. (approximately 4 mols) of the polymer, 110.4 g. (approximately 2 mols) of propionitrile and 318 ml. of 65 per cent sulfuric acid (approximately 3 mols) was stirred for 141 hours While the temperature was held at 45 to 55 C. Upon standing, three layers separated. The aqueous bottom layer was discarded. The to and middle layers were separately washed free of acid components with an aqueous sodium bicarbonate solution and were dried over calcium chloride. Distillation of the top layer revealed that it consisted only of unchanged polymer. Fractionation of the middle layer yielded 73.5 g. of a propionate ester having a boiling range of 117 to 131 C. at a pressure of 1 mm. of mercury and a refractive index (n of 1.4607. The yield of ester based on the nitrile charged was 16 per cent.

An attempt was made to improve the yield of ester by conducting the reaction at to C. for 141 hours. aqueous layer and an organic layer separated. The latter contained no ester S but was composed of olefin polymers comparable to those of the charge.

A further attempt to prepare the ester was made in essentially the same manner except that propionic acid was substituted for propionitrile. The temperature was held at 45 to 55 C. for 144 hours. Distillation of the organic layer subsequent to the removal of acidic components yielded only unchanged olefin polymers.

Example II An ester was prepared from propionitrile and an olefinic polymer consisting predominantly of dodecenes in which a molar excess of the nitrile was used. The general procedure described in Example I was followed. The reaction mixture consisted of 1 mol of olefin polymer, 2 mols of propionitrile and 2.5 mols of 65 per cent sulfuric acid and it was agitated for 143 hours at a temperature of 45 to 55 C. Distillation of the middle layer gave a 20 per cent yield of ester based on the olefin polymer charged.

The yield was slightly better in this example than in Example I, in which the nitrile was used in excess of the olefin. This may possibly be the result also of the use of the higher ratio of water (in the acid) to nitrile.

Example III An ester of an unsaturated acid (an ethyl penteneoate) was prepared from trans-S-pentenenitrile (CH3CH:CHCH2-CN) and ethylene in the presence of sulfuric acid. One mol of the nitrile and a mixture of 2 mols of ethylene in 4.4. mols of n-pentane were charged simultaneously to a reaction zone containing 1.5 mole of 65 per cent sulfuric acid. The pressure in the reaction Zone was maintained at about 300 to 350 pounds per square inch and the temperature was held at 90 to 100 C. while the mixture was continuously agitated. The emulsion was transferred to a separator where the aqueous bottom layer was drawn off, after which the organic layer was stabilized and subsequently washed with aqueous sodium bicarbonate. Distillation yielded an ester fraction corresponding to 17 per cent of th nitrile charged.

Example IV A mixture of 336 g. (approximately 2 mols) of an olefinic polymer consisting predominantly of dodecenes, 81 g. (approximately 1 mol) of trans- 3-pentene-nitrile and 159 1111 (approximately 1.5 mols) of 65 per cent sulfuric acid was stirred at 45 to 55 C. for 169 hours. These are the same relative proportions as were used in Example I and the procedure of Example I was otherwise followed. In contrast to the reaction with propionitrile in Example I, however, only two layers separated on standing. The organic layer yielded a light yellow, viscous liquid having a boiling range of 140 to 154 C. at a pressure of 2 mm. of mercury. The yield of this ester, based on the nitrile charged, was about 20 per cent of the theoretical.

Example V Cyclohexyl propionate was prepared from a mixture of 246 g. (approximately 3 mols) of cyclohexene, 55.2 g. of propionitrile (approximately 1 mol) and a solution containing 147 of concentrated sulfuric acid and 63 g. of water. The mixture was stirred and heated at reflux ternperature for hours, after which the two organic layers which separated were washed with sodium bicarbonate until free of acidic constituents and then dried. Fractionation of the dried top layer yielded 20 per cent of the theoretical amount oi ester based on the nitrile charged.

Many modifications and variations of the invention may be made without departing from the scope thereof and therefore no limitations are intended other than those defined in the appended claims.

I claim:

1. A process for the production of an ester of a carboxylic acid which comprises: reacting a nitrile with an olefin in the presence of water and a strong acid selected from the group consisting of sulfuric acid, phosphoric acid, and benzenesulfonic acid; maintaining the temperature of the reactants in the range from 40 to 200 0.; and maintaining the reactants under pressure sufiicient to maintain them in liquid phase.

2. A process for the production of an ester of a carboxylic acid which comprises: reacting a nitrile with an olefin in the presence of an aqueous solution of sulfuric acid; maintaining a stoichiometric excess of sulfuric acid over olefin; maintaining the temperature of the reac-.- tants in the range from 40 to 200 0.; and maintaining the reactants under pressure sufficient to maintain them in liquid phase.

3. A process for the production of an ester of a carboxylic acid which comprises: reacting a nitrile with an olefin in the presence of an aqueous solution of sulfuric acid having a concentration of approximately 65' per cent I'I2SO4; maintaining the temperature of the reactants in the range from 40 to 200 C.; and maintaining the reactants under pressure sufficient to maintain them in liquid phase,

4. A process for the production of a dodecyl ester of a carboxylic acid which comprises reacting a nitrile corresponding to said carboxylic acid with an olefin polymer consisting essentially of dodecenes in the presence of aqueous sulfuric acid having a concentration of approximately 65% H2804 at a temperature within the range of approximately 40 to approximately C. under pressure sufiicient to maintain liquid phase.

5. A process for the production of an ester of penteneoic acid which comprises reacting a pentenenitrile and an olefin in the presence of aque ous sulfuric acid and at a temperature within the range of approximately 40 to approximately 100 C. under pressure sufficient to maintain liquid phase.

6. A process for the'production of a cyclohexyl ester of a carboxylic acid which comprises reacting cyclohexene and the nitrile of the said carboxylic acid in the presence of aqueous sulfuric acid and at a temperature within the range of approximately 40 to approximately 100 C. under pressure sufficient to maintain liquid phase.

JOHN E. MAHAN. 

